Download The Observed Properties of Stars

The Observed Properties of Stars
Stars look like points of light in the sky.
How similar are they to the Sun?
What do we need to measure about
these stars in order to make “models”
that can be compared to our model of
the Sun?
The Observed Properties of Stars
Clicker question: Which of the following things did
we NOT need to know about the Sun in order
to compute an accurate model of its interior?
A. Chemical composition
B. Luminosity
C. Mass
D. Diameter
E. We needed to know all of the above
1
Finding the
luminosity
[Fig. 11.1/11.2]
L
F = 4πr2
[11.1]
L = 4πr2 F
• Luminosity = Energy/unit time
• But we measure flux incident
on Earth
= Energy/unit time /unit area
Î Also must know distance r
• For nearby stars, use
parallax:
[Fig. 11.2/11.3]
Stellar masses
[11.1]
• Binary stars
• Use Newton’s form of Kepler’s 3rd law:
4π 2 a3
P =
G m1 + m2
2
constant
applet
Orbits are about
center of mass.
2
Mass - Luminosity Relation
Luminosity (Lsun) Î
• Key observational result for theoretical interpretation of
different types of stars.
Mass (solar masses)Î
[Fig. 5.10]
Emitted energy
per unit surface area Î
Finding the star’s diameter
5000o
[fig 4.8]
4000o
3000o
Wavelength Î
Í Energy
• Total energy emitted per unit surface area
Stefan-Boltzmann Law:
• Total energy from whole star:
E = σ T4
Surface area
of sphere
of diameter D
L = E x (surface area) = (σ T4) x (πD2)
Luminosity
3
Two Ways:
• Thermal radiation
curve
• Spectroscopy
Emitted energy
per unit surface area Î
Taking a star’s (surface) temperature
[Fig. 5.10]
5000o
4000o
3000o
Wavelength Î
Í Energy
Sodium
Hydrogen
Calcium
Mercury
Neon
Wavelength Î
Different stars have very different-looking spectra.
G
Ionized
helium
K
Flux Æ
O
B
Low ionization & neutral atoms
A
Excited hydrogen
F
G
M
Molecules
Low ionization & neutral atoms
Wavelength Æ
What causes these differences?
4
1. Ionized atom has entirely different energy level structure
ÎEntirely different set of absorption lines.
REVIEW:
2. Excitation, ionization can also be caused by collisions between atoms.
3. Higher temperatures Î More energetic collisions Î
• electrons in higher levels on average.
• atoms in higher ionization states on average.
E = hf =
[Fig. 5.8/5.9]
hc
λ
Stellar spectral types
O
B
A
F
G
A Temperature Sequence
Higher temperatures Î
More energetic collisions Î
• electrons in higher levels on average.
• atoms in higher ionization states on average.
K
M
Excited
Relative Strength Î
Excited
[see Table 11.1]
Spectral Type
O
Temperature 30,000
B
A
10,000
F
G
K
5800
M
3500
5
O
B
A
F
G
K
Measuring the surface temperatures
of stars
Michigan State Extra-Credit Lottery!
for best OBAFGKM mnemonic.
• 3 clicker points for entering.
• 3 clicker points for best answer that can be repeated in class.
• enter by email to [email protected], before Midterm 3.
M
Excited
Relative Strength Î
Excited
[see Table 11.1]
Spectral Type
O
Temperature 30,000
O
B
B
A
10,000
F
G
K
5800
M
3500
Measuring the surface temperatures
of stars
A
F
G
K
M
Excited
Relative Strength Î
Excited
[see Table 11.1]
Spectral Type
O
Temperature 30,000
B
A
10,000
F
G
K
5800
M
3500
6